An optical recording medium may be roughly classified into a read-only medium, such as a so-called compact disc, and a medium on which signals can be recorded, such as a magneto-optical disc. With any of these optical recording media, it is desired to improve the recording density to a higher level. It is because a data volume several to more than ten times that of digital audio signals is required when recording digital video signals and because a demand is raised for reducing the size of the recording medium, such as a disc and hence the size of a product, such as a player, even when recording digital audio signals.
Meanwhile, the recording density of recording the information on the recording medium is governed by the S/N ratio of the playback signals. In the typical conventional optical recording and reproduction, the total area of a beam spot SP, which is a radiation region of the readout beam, such as the laser beam for the optical recording medium, as shown in FIG. 1, is a playback signal region. Thus the reproducible recording density is governed by the diameter D.sub.SP of the beam spot of the readout beam.
If, for example, the diameter D.sub.SP of the beam spot SP of the readout laser beam is lesser than a pitch q of a recording pit RP, two recording pits cannot be present in the spot SP, and the playback output waveform is as shown at B in FIG. 1, so that the playback signals can be read. However, if the recording pits SP are formed at a higher density, and the diameter D.sub.SP of the beam spot SP becomes larger than the pitch q of the recording pit RP, as shown at C in FIG. 1, two or more pits may be present simultaneously in the spot SP, so that the playback output waveform becomes substantially constant as shown at D in FIG. 1. In this case, these two recording pits cannot be reproduced separately, so that reproduction becomes infeasible.
The spot diameter D.sub.SP depends on the wavelength .lambda. of the laser beam and on the numerical aperture NA. It is this spot diameter D.sub.SP that governs the pit density along the scanning direction of the read-out beam or the recording track direction, or the so-called line density, and the track density conforming to the track interval between neighboring tracks in a direction at right angles to the scanning direction of the readout beam, or the so-called track pitch. The opto-physical limits of the line density and the track density are set by the wavelength .lambda. of the readout beam source and the numerical aperture NA of the objective lens and the read-out limit of 2NA/.lambda. is generally accepted as long as the spatial frequency at the time of signal reproduction is concerned. For this reason, for achieving high density of the optical recording medium, it is necessary to diminish the wavelength .lambda. of the light source of the reproducing optical system, such as a semiconductor laser, as well as to enlarge the numerical aperture NA of the objective lens.
The present Applicant has already proposed an optical recording medium in which the recordable line recording density as well as the track density may be increased without changing the spot diameter of the readout beam spot, and a method for reproducing the optical recording medium. The optical recording medium capable of reproducing the high density information in this manner may be enumerated by a magneto-optical recording medium capable of recording information signals and a variable reflectance type optical recording medium at least capable of reproducing information signals.
The above-mentioned magneto-optical recording medium includes a magnetic layer, such as a rare earth-transition metal alloy thin film, deposited on a major surface of a transparent substrate or light-transmitting substrate of e.g. polycarbonate, together with a dielectric layer and a surface protecting layer. The magnetic layer has an easy axis of magnetization perpendicular to the film surface and exhibits superior magneto-optical effect. The laser beam is irradiated from the side of the transparent substrate for recording/reproducing information signals. Signals are recorded on the magneto-optical recording medium by so-called thermo-magnetic recording in which the magnetic layer is locally heated by e.g. laser beam radiation to close to the Curie temperature to reduce the coercivity to zero in this region and a recording magnetic field is applied to this region from outside for magnetization in the direction of the recording magnetic field. The recorded signals may be reproduced by taking advantage of the magneto-optical effect, such as the so-called magnetic Kerr effect or Faraday effect, in which the plane of polarization of the linearly polarized light, such as laser beam, is rotated in the direction of the magnetization of the magnetic layer.
The variable reflectance type optical recording medium is produced by depositing a material changed in reflectance with temperature on a transparent substrate on which phase pits are formed. During signal reproduction, the readout beam is radiated on the recording medium and the reflectance is partially changed within the scanning spot of the readout beam to read out the phase pits.
In connection with the above-mentioned magneto-optical recording medium, high density reproduction or so-called high resolution reproduction is hereinafter explained.
The present Applicant has previously proposed in e.g. Japanese Patent Laid-Open Publication No. 1-143041 (1989) and Japanese Patent Laid-Open Publication No. 1-143042 (1989) a method for reproducing information signals for a magneto-optical recording medium wherein information bits (magnetic domains) are enlarged, diminished or reduced to zero for improving the playback resolution. The essential point of the technology consists in that the recording magnetic recording layer is an exchange-coupled multilayer film composed of a reproducing layer, an intermediate layer and a recording layer, and in that the magnetic domain heated by the playback light beam during reproduction is enlarged, diminished or erased at a zone of higher temperatures for diminishing the inter-bit interference during reproduction to render it possible to reproduce signals of a period lower than the light diffraction threshold. There is also proposed in the application documents of JP Patent Application No. 1-229395 (1989) a technology in which the recording layer of the magneto-optical recording medium is formed by a multilayer film including a magnetically coupled reproducing layer and a recording holding layer, the direction of magnetization is aligned in advance to an erased state, the reproducing layer is heated to a temperature higher than a predetermined temperature by irradiation of the laser light and in which magnetic signals written on the recording holding layer only in this heated state are read out while being transcribed on the reproducing layer to eliminate signal crosstalk to improve the line recording density and the track density.
The above-described high density reproducing technology may be roughly classified into an erasure type and a relief type, shown schematically in FIGS. 2 and 3, respectively.
Referring first to FIGS. 2A, 2B and 2C, the erasure type high density reproduction technique is explained. With the erasure type, the recording medium, on which information recording pits RP are exhibited at room temperature, is heated by irradiation of a laser light LB to produce an erased region ER within the beam spot SP of the radiated laser beam LB, as shown at B in FIG. 2, and the recording pit RP within a remaining region RD within the beam spot SP is read, by way of achieving reproduction with improved line density. In sum, this technique consists in that, when reading the recorded pit RP within the beam spot SP, the erased region ER is used as a mask to narrow the width d of the read-out region (playback region) RD to provide for reproduction with an increased density along the scanning direction of the laser light (track direction), that is the so-called line recording density.
The recording medium for erasure type high density reproduction has an exchange-coupled magnetic multilayer film structure composed of an amorphous rare earth for photomagnetic recording (Gd, Tb)-iron group (Fe, Co) ferrimagnetic film. In an example shown at A in FIG. 2, the recording medium has a structure in which a reproducing layer as a first magnetic film 61, an interrupting layer (intermediate layer) as a second magnetic layer 62 and a recording holding layer as a third magnetic layer 63, deposited in this order on a major surface (the lower surface in the drawing) of a transparent substrate 60 formed of e.g. polycarbonate. The first magnetic layer (reproducing layer) 61 is e.g. a GdFeCo layer with a Curie temperature T.sub.c1 &gt;400.degree. C., while the second magnetic layer (interrupting layer or an intermediate layer) 62 is e.g. a TbFeCoAl film having a Curie temperature T.sub.c2 of 120.degree. C. and the third magnetic layer (recording holding layer) is e.g. a TbFeCo layer with a Curie temperature T.sub.c3 of 300.degree. C. Meanwhile, arrow marks in the magnetic films 61 to 63 shown at C in FIG. 2 represent the direction of magnetization of the magnetic domains. H.sub.read represents the direction of the reproducing magnetic domain.
The reproducing operation is briefly explained. At an ambient temperature below a predetermined temperature T.sub.OP, the layers 63, 62 and 61 of the recording medium are magnetically coupled in the state of static magnetic coupling or exchange coupling, while the recording magnetic domain of the recording holding layer 63 is transcribed to the reproducing layer 61 by means of the interrupting layer 62. If the laser beam LB is radiated on the recording medium for raising the medium temperature, changes in the medium temperature are produced with a time delay with the scanning of the laser beam, so that a region at a temperature higher than the predetermined temperature T.sub.OP, that is the erased region ER, is shifted slightly towards the rear side of the laser spot SP in the laser scanning direction. At the temperature higher than the predetermined temperature T.sub.OP, the magnetic coupling between the recording holding layer 63 and the reproducing layer 61 disappears and the magnetic domains of the reproducing layer 61 are aligned in the direction of the reproducing magnetic field H.sub.read, with the recording pits being erased on the medium surface. A region RD of the scanning spot SP, excluding a superposed region with the region ER where the temperature is higher than the predetermined temperature T.sub.OP, substantially represents a reproducing region. That is, the laser spot SP of the laser beam is partially masked by the region ER where the temperature becomes higher than the predetermined temperature T.sub.OP, so that the small unmasked region becomes the reproducing domain RD to achieve high density reproduction.
Since pits may be reproduced by detecting e.g. the Kerr rotation angle of the beam reflected from a small reproducing region (readout region RD) in which the scanning spot SP of the laser beam is not masked by the masking region (erased region ER), the beam spot SP is equivalently increased in diameter for increasing the line recording density and the track density.
In the relief type high density reproducing technique, shown at B in FIG. 3, the recording medium in a state in which information recording pits RP are erased at ambient temperature (initialized state) is irradiated with a laser beam and thereby heated to form a signal detecting region DT, as a recording relieved region, within the beam spot SP of the laser beam, and only the recording pit RP within this signal detecting region DT is read for improving the playback line density.
The recording medium for such high density relief reproduction has a magnetic multilayer structure according to magnetostatic coupling or magnetic exchange coupling. In an example shown at A in FIG. 3, a reproducing layer 71 as a first magnetic layer, a reproduction assistant layer 72 as a second magnetic layer, an intermediate layer 73 as a third magnetic layer 73 and a recording holding layer 74 as a fourth magnetic layer are stacked sequentially on a major surface (the lower surface in FIG. 3) of a transparent substrate 70 formed of e.g. polycarbonate. The first magnetic layer (reproducing layer) 71 is formed e.g. of GdFeCo and has a Curie temperature T.sub.c1 &gt;300.degree. C., the second magnetic layer (reproduction assistant layer) 72 is formed e.g. of TbFeCoAl and has a Curie temperature T.sub.c2 .apprxeq.120.degree. C., the third magnetic layer (intermediate layer) 73 is formed e.g. of GdFeCo and has a Curie temperature T.sub.c3 .apprxeq.250.degree. C. and the fourth magnetic layer (recording holding layer) 74 is formed e.g. of TbFeCo and has a Curie temperature T.sub.c4 .apprxeq.250.degree.. The magnitude of an initializing magnetic field H.sub.in is selected to be larger than a magnetic field H.sub.cp inverting the magnetization of the reproducing layer (H.sub.in &gt;H.sub.cp) and sufficiently smaller than the magnetizing field H.sub.cr inverting the magnetization of the recording holding layer (H.sub.in &lt;&lt;H.sub.cp). The arrows in the magnetic layers 71, 72 and 73 at C in FIG. 3 indicate the direction of magnetization in each domain, H.sub.in indicates the direction of the initializing magnetic field and H.sub.read the direction of the reproducing magnetic field.
The recording holding layer 74 is a layer holding recording pits without being affected by the initializing magnetic field H.sub.in, the reproducing magnetic field H.sub.read or the reproducing temperature, and exhibits sufficient coercivity at room temperature and at the playback temperature.
The intermediate layer 73 exhibits perpendicular anisotropy less than that of the reproduction assistant layer 72 or the recording holding layer 74. Therefore, a magnetic wall may exist stably at the intermediate layer 73 when the magnetic wall is built between the reproducing layer 71 and the recording layer 74. For this reason, the reproducing layer 71 and the reproduction assistant layer 72 maintain the erased state (initialized state) in stability.
The reproduction assistant layer 72 plays the role of increasing coercivity of the reproducing layer 71 at room temperature, so that magnetization of the reproducing layer 71 and the reproduction assistant layer 72 may exist stably despite presence of the magnetic wall. On the other hand, coercivity is decreased acutely during reproduction in the vicinity of the reproduction temperature T.sub.s so that the magnetic wall confined in the intermediate wall 73 is expanded to the reproduction assistant layer 72 to invert the reproducing layer 71 ultimately to extinguish the magnetic wall. By this process, pits are caused to appear in the reproducing layer 71.
The reproducing layer 71 has a low inverting magnetic field H.sub.cp so that the domains of overall surface of the layer 71 may be aligned by the initializing field H.sub.in. The aligned domains are supported by the reproduction assistant layer 72 and may thereby be maintained stably even if there exist a magnetic field between the layer and the reproduction assistant layer 74. Recording pits are produced by the disappearance of the magnetic wall between the layer and the recording holding layer 74 during reproduction, as described above.
The operation during reproduction is explained briefly. The domains of the reproducing layer 71 and the reproduction assistant layer 72 are aligned before reproduction in one direction (in an upward direction in FIG. 3) by the initializing magnetic field H.sub.in. At this time, a magnetic wall (indicated in FIG. 3 by a transversely directed arrow) is present stably so that the reproducing layer 71 and the reproduction assistant layer 72 are stably maintained in the initialized state.
A reproducing magnetic field H.sub.read is applied in an inverse direction while a laser beam LB is radiated. The reproducing magnetic field H.sub.read needs to be in excess of the magnetic field which inverts the domains of the reproducing layer 71 and the reproduction assistant layer 72 at a reproduction temperature T.sub.RP following temperature increase by laser irradiation to cause extinction of the magnetic field of the intermediate layer 73. The reproducing magnetic field also needs to be of a such a magnitude as not to invert the direction of magnetization of the reproducing layer 71 and the reproduction assistant layer 72.
With scanning by the laser beam, temperature changes in the medium are produced with a delay, so that the region whose temperature exceeds a predetermined reproducing temperature T.sub.RP (recording relieved region) is shifted slightly from the beam spot SP towards the rear side along the scanning direction. With the temperature above the predetermined reproducing temperature T.sub.RP, coercivity of the reproduction assistant layer 72 is lowered, so that, when the reproducing magnetic field H.sub.read is applied, the magnetic wall is caused to disappear so that the information of the recording holding layer 74 is transcribed on the reproducing layer 71. Thus a region within the beam spot SP which does not reach the reproducing temperature T.sub.RP is masked and the remaining region within the beam spot SP becomes the signal detecting region (reproducing region) DT which is the recording relieved region. High density reproduction may be achieved by detecting e.g. the Kerr rotation angle of a plane of polarization of the reflected beam from the signal detecting region DT.
That is, the region within the beam spot SP of the laser beam LB which does yet not reach the reproduction temperature T.sub.RP is a mask region in which recording pits are not displayed, while the remaining signal detecting region (reproducing region) DT is smaller in area than the spot diameter, so that the line recording density and the track density may be increased in the same manner as described above.
There is also devised a high density reproducing technique consisting in a combination of the erasure type and the relief type. With this technique, the laser beam is radiated to the recording medium in an initialized state thereof in which recording pits are extinct at room temperature. This heats the recording medium for forming a recording relieved region at a site slightly deviated towards the rear side of the beam spot of the radiating laser beam, simultaneously forming an erased region of a higher temperature within the recording relieved region.
In the specification and the drawings of our co-pending JP Patent Application No. H 3-418110 (1991), there is proposed a signal reproducing method for a magneto-optical recording medium wherein a magneto-optical recording medium having at least a reproducing layer, an intermediate layer and a recording holding layer is employed, a laser beam is radiated and a reproducing magnetic field is applied to the reproducing layer, a temperature distribution generated by the laser radiation is utilized to produce a region where an initialized state is maintained, a region to which the information of the recording holding layer is transcribed and a region the domains of which are aligned in the direction of the reproducing magnetic field, in a field of view of the lens, to produce a state equivalent to optically masking the field of view of the lens to increase the line recording density and the track density as well as to assure satisfactory frequency characteristics at the time of reproduction, there being no risk that, even if the reproducing power is fluctuated, the region of transcription of the information of the recording holding layer be diminished or enlarged.
According to the above-described high density reproducing technique employing such magneto-optical recording medium, only the read region RD, which is in effect the signal reproducing region, or the recording pit RP within the signal detecting region DT, is read within the beam spot SP. Since the size of the read region RD or the signal detection region DT is smaller than the size of the beam spot SP, the distance between adjacent pits in the directions along and at right angles to the laser beam scanning direction may be reduced to raise the line density and the track density to increase the recording capacity of the recording medium.
Meanwhile, with the above-described method for reproducing the high-density information, even although the external reproducing magnetic field is constant and the laser light power is constant, the size of the region RD of FIG. 2 or that of the region DT of FIG. 3, as the reproducing region, is fluctuated with changes in the temperature of the recording medium, such as the magneto-optical disc, brought about by changes in ambient temperature.
For example, with the erasure type reproducing method, explained in connection with FIG. 2, if the temperature of the recording medium, such as the magneto-optical disc, is high, the state of temperature distribution shows a shift towards a higher temperature, as shown by a curve a at B in FIG. 4, so that the erased region exceeding the Curie temperature T.sub.c (mask region) is as shown at ER.sub.HT at A in FIG. 4, so that the effective reading region (reproducing region) RD is diminished.
On the other hand, if the temperature of the recording medium is low, the state of temperature distribution shows a shift towards a lower temperature, as shown by a curve b at B in FIG. 4, so that the erased region exceeding the Curie temperature T.sub.c (mask region) is as shown at ER.sub.LT at A in FIG. 4, so that the effective reading region (reproducing region) RD is diminished.
With the relief type, as will become apparent from its operating principle, if the temperature of the magneto-optical recording medium is high, the reproducing region is increased, whereas, if the temperature of the magneto-optical recording medium is low, the reproducing region is diminished.
As described above, if, with the erasure type reproducing method or with the relief type reproducing method, the effective reproducing region is fluctuated, there is a risk that stable reproduction with a high S/N ratio cannot be achieved.
The same may be said when reproducing a variable reflectance optical recording medium by way of high density reproduction or ultra high resolution reproduction. That is, since the portion within the readout beam which is changed in reflectance is changed in size with changes in the medium temperature, the high reflectance portion, which is in effect the reproducing region, is fluctuated in size with the medium temperature, so that stable reproduction can occasionally not be achieved.
In view of the above-described status of the art, it is an object of the present invention to provide a method for reproducing an optical recording medium in which, even although the temperature of the magneto-optical recording medium of the variable reflectance type optical recording medium is changed, the size of the effective reproducing region may be maintained constant to assure stable reading of information signals.